Very quickly he went from being a happy, walking child to little more than a rag doll — without the strength to feed himself, or write, let alone walk.

"The moments when I really struggled it was really, really bad," he says.

"Even this year I get sad seeing other people playing around or doing sport. That's one of the things that gets to me, especially at school."

Saddest of all was the news that for most sufferers, mitochondrial diseases have no cure, or any effective treatment.

At best, Brandon would never get out of the wheelchair.

"That was hard," his mother says, struggling to hold back tears.

"Watching him fade away. Being the only boy in the family, you know, I wanted to see my little man get out and about and have fun and kick a ball. And he wasn't able to do that. And it was heart-breaking."

Brandon re-diagnosed with treatable genetic disease

Then, two years ago, when Brandon was 10, Professor Christodoulou called him in to Westmead Children's Hospital to give him life-changing news.

Doctors had been wrong all along. Revolutionary advances in genetic sequencing proved he did not have mitochondrial disease at all.

"In recent years the capacity to read the genetic code of every single gene — all 20,000 of them in the human body — has reached a point where it is now efficient, accurate, cost-effective to be able to do this," he says.

The new technique is called "next generation sequencing" — and where previously it took weeks or months to analyse the code of a single gene, today laboratory computers can decode all 20,000 genes in one go.

Almost immediately Westmead Children's Hospital researchers could pinpoint which one of Brandon's genes had a mutation.

Professor Christodoulou illustrates how the technique works on a chart.

"So here in the unaffected individual we have an 'A'. Here in the affected individual we have a 'G'. And that's precisely where the mistake is," he says.

Doctors using next generation sequencing discovered in fact that Brandon had congenital myasthenia — a different genetic disease which also affects the muscles.

But although incurable, it is not usually fatal and can be treated with medication.

"The name of the gene that we found the mistakes in is called COLQ, and it has a completely different role," Professor Christodoulou says.

"It has nothing to do with mitochondrial energy production.

"What it is involved in is co-ordinating the communication of nerve cells with the muscle, so that the muscle, when it receives an impulse from a nerve cell, it contracts and relaxes appropriately.

"So the problem with the COLQ mistakes is that this process couldn't be co-ordinated properly. And that's what actually led to his progressive problems."

Drug restores muscle strength, Brandon walks

Finding that one gene in 20,000 has transformed Brandon's life. Videos taken by Westmead staff show him walking.

A simple drug quickly restored some of his muscle strength. As quickly as he had deteriorated as a toddler, he suddenly began making huge strides.

"We noticed it straight away. By the end of that week he got up off that bed and he walked," she says, wiping away tears.

"That was unreal. I'll never forget that day. I was so happy for him and ... I just knew from that he was going to grow, he was going to enjoy his life that much more than what he had before. And he has."

Professor Christodoulou says it was a very gratifying outcome for doctors.

"We've all shed tears from time to time, both happy and sad tears. And this was certainly one of the happy tears times," he said.

'The more mysterious an illness, more likely it's genetic'

Westmead Children's Hospital now hopes to invest in more laboratory facilities to help diagnose and treat countless more rare children's diseases.

Geneticists say "next generation" sequencing is a revolution in medicine, and investigations being pioneered here foreshadow new tests and one day drugs for patients everywhere.

"The more mysterious an illness is and the more systems in the body that are affected, the more likely you're going to be potentially looking at a genetic cause," Professor Christodoulou says.

"But our ideal goal is of course to be able to identify the genetic cause with a view to being able to implement specific treatments."

While Brandon will probably need a wheelchair for life, visits to doctors like Professor Christodoulou will become far less frequent.

And with high school from next year, he is determined to grab hold of new opportunities.

"I'm outside a lot more. I'm doing things at school a lot more. A big change I guess in six months," he says.

His mother says she is optimistic her son can live a long, happy and fulfilling life.

"I just want Brandon to follow his dreams, whatever they may be," she says.

"I think I need to take a step back and let him spread his wings as they say and just go for it."